Give the opportunity to alter the default value of a Flag

[malcommac]

Feature Request

Give the opportunity to alter the default value of a Flag.
Sometimes you may need to alter the default value specified inside the property wrapper signature. This happens if you want to define a single collection file used in several targets of your project where one or more flags have different default values.
In this case you can avoid to replicate the same collection blueprint but just change the default value.

A new call setDefaultValue() is therefore provided to allows this change.

Q	A
New Feature	yes
BC Break	no

[malcommac]

Sometimes you may want to alter the default value of a Flag set via the annotation default parameters.
This is true when, for example, you have different target of your product with different values for some flag and you would avoid creating duplicate files for each Flags Collection blue print.

In this case you can define your own FlagsCollections and use the setDefaultValue() on each different flag to setup your own value.

Consider this example:

struct Flags: FlagCollectionProtocol {
    @FlagCollection(default: 100, description: "...")
    var flagA: Int

    @Flag(default: false, description: "...")
    var flagB: Bool
}

public func setupFlagsByTarget {
    self.loader = FlagsLoader(Flags.self, provider: [...])

    #if TARGET_A
      // Target A only differ for a 200 default value for flagA
      loader.$flagA.setDefault(200)
    #endif
    
    #if TARGET_B
      // Target B only differ in flagB which is false by default
      loader.$flagB.setDefault(false)
    #endif
    
    #if TARGET_C
      // Target C has the same false for flagB but a different value for flagA
      loader.$flagA.setDefault(50)
    #endif
}